"In case you haven't seen a sales report these days, February [month-to-date] sales are a total disaster," Jerry Murray, Walmart's vice president of finance and logistics, said in an internal email leaked to Bloomberg. "The worst start to a month I have seen in my 7 years with the company."

In another email, a corporate exec asked: "Have you ever had one of those weeks where your best-prepared plans weren't good enough to accomplish everything you set out to do? Well, we just had one of those weeks here at Walmart U.S. Where are all the customers? And where's their money?"

One prevailing theory: Blame the Jan. 1 expiration of a temporary 2 percent payroll tax reduction, which lawmakers agreed to as part of the deal that avoided the "fiscal cliff." As a result, pretty much every salaried worker in America is seeing a smaller number on their paychecks — and that hurts retail giants like Walmart.

"It's amazing how little attention the payroll-tax increase got at the time — maybe because so few of the players and observers involved could imagine how much difference $15 out of the weekly paycheck of someone earning $40,000 a year could make," says George Packer at The New Yorker.

It turns out Walmart's customers may have "needed that $15 more than most Washington politicians" might have thought, Packer says. "America's vast population of working poor can only get so poor before even Walmart is out of reach."

But whether the lackluster sales are temporary or an omen of things to come, industry observers should keep an eye on Walmart.

"Walmart makes up such a huge chunk of the U.S. economy — 2.3 percent of GDP in 2006 — that many analysts look to it as a key bellwether," says Brad Plummer at TheWashington Post. "Pay attention to Walmart. They often know something the rest of us don't."